1. Field of the Invention
The present invention relates generally to pulse oximetry and, more particularly, to sensors used for pulse oximetry.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In the field of medicine, doctors often desire to monitor certain physiological characteristics of their patients. Accordingly, a wide variety of devices have been developed for monitoring physiological characteristics of a patient. Such devices provide doctors and other healthcare personnel with the information they need to provide the best possible healthcare for their patients. As a result, such monitoring devices have become an indispensable part of modern medicine.
One technique for monitoring certain physiological characteristics of a patient is commonly referred to as pulse oximetry, and the devices built based upon pulse oximetry techniques are commonly referred to as pulse oximeters. Pulse oximetry may be used to measure various blood flow characteristics, such as the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and/or the rate of blood pulsations corresponding to each heartbeat of a patient. In fact, the “pulse” in pulse oximetry refers to the time varying amount of arterial blood in the tissue during each cardiac cycle.
Pulse oximeters typically utilize a non-invasive sensor that transmits light through a patient's tissue and that photoelectrically senses the absorption and/or scattering of the transmitted light in such tissue. One or more of the above physiological characteristics may then be calculated based upon the amount of light absorbed or scattered. More specifically, the light passed through the tissue is typically selected to be of one or more wavelengths that may be absorbed or scattered by the blood in an amount correlative to the amount of the blood constituent present in the blood. The amount of light absorbed and/or scattered may then be used to estimate the amount of blood constituent in the tissue using various algorithms. Changes in the amount of arterial blood in the tissue during a blood pressure pulse may change the amount and character of the light detected by the sensor's photodetector.
The quality of the pulse oximetry measurement depends in part on the concentration of arterial blood relative to other tissue structures in the portion of the tissue illuminated by the sensor and in part on the magnitude of the pulsatile changes in the amount of blood in the tissue. Pulse oximetry techniques typically utilize a tissue site that is well perfused with blood, such as a patient's finger, toe, or earlobe, on which to place the sensor. Although these sites are usually well perfused, blood flow to the sensor site may be restricted due to the effects of ambient temperature, systemically acting vasoconstricting drugs in the patient's blood stream, or low blood pressure. The accuracy and reliability of physiological measurements can be affected by the amount of blood perfusion, as well as by the distribution of blood flow within a tissue site. Furthermore, physiological differences from patient to patient, or even from digit to digit, may cause unintended variations in the measurements provided.